[Bug c/96527] New: __builtin_va_arg_pack_len produces error in documented sample code

[bruno at clisp dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96527

            Bug ID: 96527
           Summary: __builtin_va_arg_pack_len produces error in documented
                    sample code
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bruno at clisp dot org
  Target Milestone: ---

Created attachment 49024
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49024&action=edit
Test case

The only documented example of __builtin_va_arg_pack_len, in
https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Constructing-Calls.html ,
produces an error "invalid use of '__builtin_va_arg_pack_len ()'" when actually
used.

Test case foo.c is attached. GCC 5.5.0, 6.5.0, 7.5.0, 8.4.0, 9.3.0, 10.2.0 all
produce an error:

$ gcc -O2 -Wall -S foo.c
foo.c: In function 'myopen':
foo.c:7:7: error: invalid use of '__builtin_va_arg_pack_len ()'
    7 |   if (__builtin_va_arg_pack_len () > 1)
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~